1. Technical Field of the Invention
The present invention relates generally to the field of photonic crystals; and, more particularly, to a three-dimensional photonic crystal add-drop filter.
2. Description of Related Art
Photonic crystals (PC) are periodic dielectric structures that can prohibit the propagation of light in certain frequency ranges (see J. D. Joannopoulos, R. D. Meade and J. N. Winn, Photonic Crystals, Princeton University Press, Princeton, N.J., 1995). More particularly, photonic crystals are structures that have spatially periodic variations in refractive index; and with a sufficiently high refractive index contrast, photonic bandgaps can be opened in the structure""s optical transmission characteristics. The term xe2x80x9cphotonic bandgapxe2x80x9d as used herein and as is commonly used in the art is a frequency range in which propagation of light through the photonic crystal is prevented. In addition, the term xe2x80x9clightxe2x80x9d as used herein is intended to include radiation throughout the electromagnetic spectrum, and is not limited to visible light.
Two-dimensional photonic crystal slabs are known that comprise a two-dimensional periodic lattice incorporated within a slab body. In a two-dimensional photonic crystal slab, light propagating in the slab is confined in the direction perpendicular to the faces of the slab via total internal reflection. Light propagating in the slab in directions other than perpendicular to the slab faces, however, is controlled by the spatially periodic structure of the slab. In particular, the spatially periodic structure causes a photonic bandgap to be opened in the transmission characteristics of the structure within which the propagation of light through the slab is prevented. Specifically, light propagating in a two-dimensional photonic crystal slab in directions other than perpendicular to a slab face and having a frequency within a bandgap of the slab will not propagate through the slab; while light having frequencies outside the bandgap is transmitted through the slab unhindered.
It is known that the introduction of defects in the periodic lattice of a photonic crystal allows the existence of localized electromagnetic states that are trapped at the defect site, and that have resonant frequencies within the bandgap of the surrounding photonic crystal material. By arranging these defects in an appropriate manner, a waveguide can be created in the photonic crystal through which light having frequencies within the bandgap of the photonic crystal (and that would normally be prevented from propagating through the photonic crystal) is transmitted through the photonic crystal.
Three-dimensional photonic crystals that have spatial periodicity in three dimensions, and that can prevent the propagation of light having a frequency within the crystal""s bandgap in all directions, are also known. For example, a known three-dimensional photonic crystal apparatus comprising dielectric elements stacked layer by layer is illustrated in FIG. 1 (also see U.S. Pat. Nos. 5,335,240 and 5,406,573 and K. M. Ho, et al., xe2x80x9cSolid State Commun.xe2x80x9d, 89, 413, 1994).
The three-dimensional photonic crystal apparatus illustrated in FIG. 1 is generally designated by reference number 10 and comprises a plurality of layers of elements arranged one on top of another. In FIG. 1, three-dimensional photonic crystal apparatus 10 comprises twelve layers 12-1 to 12-12; however, twelve layers is intended to be exemplary only as the apparatus can comprise any desired plurality of layers.
Each layer 12-1 to 12-12 comprises a plurality of elements arranged to be parallel to and equally spaced from one another. In addition, the plurality of elements in each layer are arranged perpendicular to the elements in an adjacent layer.
In FIG. 1, the elements comprise rods, and layers 12-1, 12-3, 12-5, 12-7, 12-9 and 12-11 each comprise a plurality of rods 14 arranged in a direction parallel to the x-axis of the apparatus (as shown in FIG. 1); and layers 12-2, 12-4, 12-6, 12-8, 12-10 and 12-12 each comprise a plurality of rods 16 arranged in a direction parallel to the y-axis of the apparatus. In addition, as shown in FIG. 1, in every other layer, the rods are laterally displaced with respect to one another by an amount equal to one-half the spacing between the rods in a layer. Specifically, in FIG. 1, the rods in layers 12-3, 12-7 and 12-11 are aligned with respect to one another along the y-axis, but are laterally displaced, along the y-axis, from the plurality of rods in layers 12-1, 12-5 and 12-9. Also, the rods in layers 12-2, 12-6 and 12-10 are aligned with respect to one another along the x-axis, but are laterally displaced, along the x-axis, from the plurality of rods in layers 12-4, 12-8 and 12-12.
The three-dimensional photonic crystal apparatus 10 of FIG. 1, can be described as comprising a photonic crystal having a three-dimensional array of unit cells therein in which a xe2x80x9cunit cellxe2x80x9d is defined as a cell having dimensions in the x and y directions equal to the spacing between the rods in the layers, i.e., the dimensions 41 and 42 in FIG. 1; and a dimension in the z-direction equal to the thickness of four layers, i.e., the dimension 44 in FIG. 1.
In the three-dimensional photonic crystal apparatus illustrated in FIG. 1, rods 14 and 16 comprise dielectric rods of a material having a high dielectric constant, e.g., alumina, surrounded by a material having a low dielectric constant, e.g., air.
Wave division multiplexing is a process that permits the transmission capacity of an optical communications system to be increased. In particular, in a wave division multiplexer (WDM) system, information is transmitted using a plurality of optical carrier signals, each carrier signal having a different optical wavelength. By modulating each carrier signal with a different one of a plurality of information signals, the plurality of information signals can be simultaneously transmitted through a single waveguiding device such as a single optical fiber.
For a WDM system to function properly, the system must have the capability of extracting a carrier signal at a certain wavelength from one waveguide and adding the signal at that wavelength to another waveguide so as to redirect the path through which the extracted carrier signal travels.
FIG. 2 is a block diagram that schematically illustrates components of a WDM communications system. The system is generally designated by reference number 20, and includes a signal source 22 that transmits a plurality of carrier signals at different optical wavelengths through an optical fiber or other waveguiding device 24. The optical fiber 24 is connected to an extraction device 26 that is capable of extracting one or more of the carrier signals carried by the optical fiber 24 and redirecting the extracted signal or signals to another optical fiber or waveguiding device 28. The remaining carrier signals carried by the optical fiber 24 are transmitted through the extraction device 26 to an optical fiber 30 or the like. The carrier signals carried by optical fibers 28 and 30 are then further processed by processing structure not illustrated in FIG. 2.
Add-drop filters are commonly used in optical communications circuits to extract light of a particular wavelength from one waveguide and direct the extracted light to another waveguide. In effect, an add-drop filter allows light of one wavelength to be dropped from one path in an optical communications circuit and added to another path in the circuit.
Known add-drop filters, however, are not fully satisfactory for use as an extraction device in a WDM system. For example, in one known configuration, light propagates through conventional high dielectric waveguides and the cavities between the waveguides-are micro-rings (see B. E. Little and S. T. Chu, Toward very large-scale integrated photonics, Optics and Photonics News, page 24, November 2000). Since the propagation is through the high dielectric material, however, internal losses and dispersion of the material may critically affect the results. The results will be even more critically affected as the power of the light waves increase in future WDM systems.
In U.S. Pat. No. 6,130,969, configurations are described in which waveguides and cavities are created in two-dimensional photonic crystals having what may be considered as infinitely long dielectric rods. These configurations, however, are difficult to fabricate with dimensions commensurate with optical communications wavelengths. In two-dimensional photonic crystals having finite length dielectric rods, there is a problem with the confinement of the light along the axes of the rods. Also, the photonic bandgap appears only in one polarization; and, as a result, the configurations are inherently multimode.
There is, accordingly, a need for an extraction device for use in WDM communications systems and for other applications that is capable of extracting and redirecting one or more wavelengths from an optical signal that includes a plurality of wavelengths, that can be made single mode so as to avoid mixing of two polarizations and in which internal losses and dispersion of high refractive index materials used in the extraction device are not so important.
The present invention provides a three-dimensional photonic crystal add-drop filter apparatus that is capable of extracting and redirecting one or more wavelengths from an optical signal that includes a plurality of wavelengths.
A three-dimensional photonic crystal add-drop filter apparatus according to the present invention comprises a three-dimensional photonic crystal having a first waveguide for transmitting light having a frequency within a bandgap of the three-dimensional photonic crystal, and a second waveguide. A resonant cavity couples light from the first waveguide to the second waveguide for extracting at least one wavelength of the light transmitted in the first waveguide and redirecting the extracted light to the second waveguide.
The resonant cavity modifies the transmission characteristics of the first waveguide by creating one or more transmission zeros that comprise narrow frequency ranges within the bandgap of the photonic crystal at which light that is otherwise capable of being transmitted through the first waveguide is prevented from propagating through the first waveguide, i.e., is xe2x80x9cfilteredxe2x80x9d out of the first waveguide. By coupling light from the first waveguide to the second waveguide through the resonant cavity, the light that is prevented from propagating through the first waveguide is redirected to the second waveguide. As a result, a three-dimensional photonic crystal add-drop filter apparatus is provided that is capable of removing light of one or more wavelengths from the first waveguide and redirecting the removed light to the second waveguide.
According to an embodiment of the invention, the three-dimensional photonic crystal comprises a plurality of layers arranged one above another. Each of the plurality of layers comprises a plurality of elements that are parallel to and spaced from one another, and the plurality of elements in each layer are arranged at an angle greater than zero degrees with respect to the plurality of elements in an adjacent layer. The first waveguide comprises a first region of defects in an element in a layer of the plurality of layers and having a light input and a light output, and the second waveguide comprises a second region of defects in a portion of an element in a layer of the plurality of layers and having a light output. The resonant cavity comprises a third region of defects in one or more layers to couple the first and second regions of defects.
According to another embodiment of the present invention, the first and second waveguides are in different layers of the plurality of layers such that the light extracted from the first waveguide and redirected to the second waveguide exits the second waveguide in a different x-y plane than the light in the first waveguide. This capability of redirecting light from one layer of the three-dimensional photonic crystal apparatus to another layer of the apparatus provides a designer with substantial flexibility when the apparatus is incorporated in an optical circuit.
According to another embodiment of the invention, the plurality of elements in each layer comprise a plurality of dielectric rods, such as alumina rods, and the plurality of dielectric rods in one layer are arranged perpendicular to the plurality of rods in adjacent layers; and, in addition, the plurality of rods in every other layer are laterally displaced with respect to one another.
A three-dimensional photonic crystal apparatus according to embodiments of the present invention provides a fully three-dimensional photonic bandgap. Accordingly, total internal reflection is not needed to confine the light. Instead, the light is confined in the low dielectric region of the photonic crystal (e.g., in air) such that the effects of internal losses and dispersion of the high refractive index medium (i.e., the elements) are not so important. In addition, the waveguides can be made single mode such that only one polarization is allowed to propagate and there is no mixing of the two polarizations.
A three-dimensional photonic crystal add-drop filter apparatus according to the present invention can be designed to precisely control the wavelength of light extracted from an optical signal. The apparatus is, accordingly, particularly suitable for use as an extraction device in WDM communications systems and in other applications that require the extraction of one or more wavelengths of light from a signal that includes a plurality of wavelengths.
Yet further advantages, specific details and other embodiments of the present invention will become apparent hereinafter in conjunction with the following detailed description of exemplary embodiments of the invention.